This invention relates to DC static switching circuits particularly for use in electrical power systems.
As general background to the present invention, reference is made to an article by the present inventor entitled "Three Types of Solid State Remote Power Controllers" appearing in Proceedings of the IEEE "Power Electronics Specialists Conference", June 1975. As is described therein, it is known to provide a transistorized DC static switch with a current limiting feature wherein the switching transistor base is connected in a servo loop with an operational amplifier to control maximum load current. Reference is made particularly to FIG. 8, and the discussion relating thereto, in said article. This form of current limiting provides excellent current control which is independent of supply voltage variations. Such circuits suffer the disadvantage, however, of having undesirable power dissipation for varying load current conditions below the current limiting level.
In applications in which solid state power controllers are particularly suitable, such as airborne applications, efficiency with minimal cost is an important requirement. The former circuit provides full base drive current to the switching transistor continuously during all normal non-overload conditions. That is, the base drive current is independent of load current for levels below current limiting. At full rated load current, the drive losses are approximately ##EQU1## or about 1% of rated load, where I.sub.L and V.sub.L are load current and supply voltage, respectively, and .beta. is the gain of the transistor power switch (approximately 100). This results in an efficiency of no more than about 99% at full load. Allowing for other losses, such as saturation voltage drop, the total losses approach 2 to 3% of full load delivered power giving an actual efficiency of 97 to 98% at rated load for a typical 28 volt DC system. This performance meets normal full load efficiency requirements, but at reduced load current levels the efficiency becomes poor.
At one-tenth of rated load current, for example, the drive losses remain at fixed magnitude, and are 10 times greater from the efficiency standpoint than at full load. These losses now represent about 10% of delivered load power giving a maximum attainable efficiency of less than 90%. Normal operation of solid state power controllers requires frequent use at below full load current and therefore this extra loss of efficiency becomes a significant factor which is desirably to be avoided.